The present invention relates to communication systems, and more particularly to the processing of multipath propagation delay information for use in mobile communication systems.
Modern telecommunications systems, such as cellular telecommunications systems, rely on digital technology for the representation and transmission of information, such as audio information. Any of a number of modulation techniques is used to impose the digital information onto a radio frequency signal, which is then transmitted from a sender's antenna, and which is received by a receiver's antenna. Ideally, the receiver would merely perform a reverse of the modulation process to recover the digital information from the received signal.
In practice, however, the transmitted signal is often distorted by the channel (i.e., the air interface) between the transmitter's antenna and the receiver's antenna. For example, a main ray of a transmitted signal may take a direct route between the transmitting and receiving antennas, but other rays may take indirect routes, such as reflecting off of various objects (e.g., buildings, mountains) in the environment prior to being received by the receiver's antenna. This effect is often called “multipath propagation” of the signal. These indirect paths can take longer for the signal to traverse than the direct path. Consequently, signals representing the same information emanating from the same source may arrive at the receiver at
different times. The various paths between the transmitter and the receiver subject the signal to varying amounts of attenuation, so they are not all received at the same signal strength. Nonetheless, they are typically received at sufficiently high power levels to cause an effect wherein, at any moment, a received signal includes a present signal (representing a present piece of desired information) plus one or more delayed components from previously transmitted signals (each representing an earlier piece of information). This type of signal distortion is often called Inter-Symbol Interference (ISI).
To counteract ISI, a receiver typically employs an equalizer, which demodulates the signal in a way that utilizes a model of the channel (also referred to as an “estimate” of the channel). The channel estimate is typically generated from another component in the receiver, called a channel estimator. A channel estimator relies on a received signal including a portion, often called a “training sequence”, that contains a predefined sequence of 1's and 0's known to have been transmitted by the transmitter. By comparing an actually received training sequence portion of a signal with an expected training sequence, the channel estimator is able to construct a model of the channel that can be used by the equalizer when it attempts to demodulate a portion of the received signal that includes unknown information.
While one might consider ISI to be a detrimental affect of multipath propagation, the phenomenon of multipath propagation may itself be applied to the benefit of a communications system, such as to combat fading in a Code Division Multiple Access (CDMA) system. CDMA is a channel access technique that allows signals to overlap in both time and frequency. CDMA is a type of spread spectrum communication technique, which has been around since the days of World War II. Early applications were predominantly military-oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications because spread spectrum communications provide robustness against interference, and allow for multiple signals to occupy the same radio band at the same time. Examples of such commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a CDMA system, each signal is transmitted using any of a number of spread spectrum techniques. In one such variation of CDMA, called “Direct Sequence CDMA” (DS-CDMA) (e.g., Wideband-CDMA—“WCDMA”), the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. This permits the same broadband frequency channel to be re-used in every adjacent cell. Typically, the signature sequence data are binary, thereby providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
A plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and de-spread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate respective ones of two carriers at the same frequency, but ninety degrees out of phase with respect to one another.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or −1, sometimes denoted “0” or “1 ”. The signature sequence usually comprises N bits, and each bit of the signature sequence is called a “chip”. The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, such as a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. Only the real part of the correlation value is computed. When a large positive correlation results, a “0” is detected; when a large negative correlation results, a “1” is detected.
In order to optimally detect the transmitted signal, the strongest rays of the multipath propagated signal must be combined in an appropriate way. This is usually done by the RAKE receiver, which is so named because it “rakes” different paths together. A RAKE receiver uses a form of diversity combining to collect the signal energy from the various received signal paths (or rays). The term “diversity” refers to the fact that a RAKE receiver uses redundant communication channels so that when some channels fade, communication is still possible over non-fading channels. A CDMA RAKE receiver combats fading by detecting the echo signals individually, and then adding them together coherently.
In order to collect the different rays, the RAKE receiver comprises a number of so-called “fingers”, each configured to receive the information from a differently delayed version of the received signal. The receiver can use a searcher to determine those delays associated with the strongest signal energy. Briefly, a searcher operates by correlating differently delayed versions of a chip sequence known to be present in the received signal against the received signal. The delays associated with the highest correlation values are then stored as the “delay profile” of this channel. It is important that the RAKE receiver configure each of its fingers to use a corresponding one of the strongest taps (paths). If the receiver does not use the strongest taps, the receiver will ask for more power and thereby increase the interference experienced by the other receivers. The overall interference is minimized when each of the receivers uses the least amount of power possible. It will be apparent, then, that in a receiver such as a RAKE receiver, it is important for the delay profile to reflect, as much as possible, present conditions of the receiver. However, as the receiver moves from place to place, the delay profile will change in correspondence with the changed terrain over which the signal has propagated. Thus, it is desired to provide improved methods and apparatuses for maintaining an accurate delay profile.
The phenomenon of multipath propagation can also be advantageously applied in the mobile communication device's quest to identify new cells in its vicinity. When moving with a mobile communication device in a mobile communications system, the device constantly needs to look for new cells, with potentially better transmission conditions. This process is called the cell search. For example, in accordance with the present standard for WCDMA mobile telephone systems, the device keeps track of 21 cells simultaneously, of which 9 are intra-frequency cells and two other frequencies (inter-frequency) containing at most 6 cells each. As the device moves around, this list of monitored cells needs to be updated to reflect present conditions.
At a high level, the cell search can be viewed as a process whereby the device processes a received signal to determine its source. More particularly, the cell search comprises three phases, as illustrated in the flow diagram of FIG. 1. In the first phase (phase 1), the slot boundary (i.e., time slot boundary) is found; in the second phase (phase 2), the slot boundary is used to find the frame boundary; and in the third phase (phase 3), knowledge of the frame boundary enables the cell's scrambling code to be determined. The scrambling code identifies the cell.
Looking first at phase 1, the search to find the slot boundary is aided by the fact that all cells use the same primary synchronization code. Thus, to find the slot boundary, a known primary synchronization code is correlated against the received signal for a range of delay values that span the duration of a slot (e.g., in WCDMA, over at least 2560 chips) (step 101). This generates a correlation value for each tested delay.
The primary synchronization code appears once in each time slot contained in a transmitted frame. (In WCDMA, each frame includes 15 time slots). In order to improve performance (e.g., to mitigate the effects of a short fade in the signal), this correlation process is repeated for each of a number of successively received time slots. That is, if the duration of a time slot is Ts, then for each delay value Td, a correlation is performed at a position Tcorr(n)=Td+nTs, where n=0, . . . , Ntest—slots−1 and Ntest—slots is the number of slots to be tested. For example, in a WCDMA system, one might perform the at least 2560 test correlations for each of the 15 slots known to be present in a frame.
For each tested delay value, the resultant correlation values are then accumulated (step 102). The maximum accumulated value is then taken as the slot boundary for a cell (step 103).
Knowledge of the slot boundary does not, by itself, inform the device of what the frame boundary is, because as mentioned earlier, each frame includes more than one slot. Turning now to phase 2, the frame boundary may be found by using the just-determined slot boundary in conjunction with a set of known secondary synchronization codes. In systems such as WCDMA, each frame includes a secondary synchronization code positioned at a known location within the frame. While the particular secondary synchronization code for the cell being “searched” is not known to the device doing the testing, the set of predefined secondary synchronization codes is. Thus, the device may hypothesize that each of the, for example, 15 slot boundaries is the start of the frame, and for each of these hypothesized frame boundaries, determine where the secondary synchronization code should be. For each of the hypothesized secondary synchronization code locations, each of the secondary synchronization codes known to be in use in the communication system is correlated against the received signal. This generates, for each hypothesized secondary synchronization code location, a correlation value. The highest correlation value from among all of the performed correlations is taken as an indicator of the secondary synchronization code location (step 104). Since the secondary synchronization code location is defined to have a predefined offset from the frame boundary, the frame boundary may be easily determined. Knowledge of which hypothesized secondary synchronization code is associated with the highest correlation value also informs the device what the secondary synchronization code is for the cell being “searched”.
Further in accordance with communication system standards such as those set forth for WCDMA, each secondary synchronization code is, itself, associated with a particular set of scrambling codes. The scrambling code is located once in each frame at a known offset from the frame boundary. Thus, in phase 3 of the cell search process, the scrambling code for the cell is found by correlating each of the scrambling codes associated with the known secondary synchronization code against the received signal at the known offset from the frame boundary. The highest correlating scrambling code is then taken to be the scrambling code for this “searched” cell (step 105).
As is readily apparent from the above, there is quite a bit of processing involved in discovering a cell. However, in this process there is nothing to prevent the slot boundary of an already known cell from being detected when it is necessary to identify multiple cells within the same carrier frequency (a common occurrence in CDMA systems). Thus, it is desired to provide cell search methods and apparatuses that avoid unnecessary processing steps.
It is therefore an object of the present invention to provide methods and apparatuses that address one or more of the above-described problems.